Gentoo's Bugzilla –
Attachment 522628 Details for
Bug 649746x11-libs/libdrm-2.4.91 - In file included from /usr/include/sys/sysctl.h:63:0, from ../libdrm-2.4.91/xf86drmMode.c:50: /usr/include/bits/sysctl.h:19:3: error: #error "sysctl system call is unsupported in x32 kernel"